In accordance with development of portable apparatuses having smaller size, lighter weight, and higher performance, increasing demand has arisen for a secondary battery having high energy density and a secondary battery of high capacity. In view of such tendency, most small-sized portable apparatuses, such as cellular phones and video cameras, have been employing a lithium ion battery using a non-aqueous electrolytic solution, or a non-aqueous lithium secondary battery such as a lithium polymer battery, both of which exhibit high energy density and high voltage. Such a lithium secondary battery employs, as a positive electrode material, a metal oxide, such as lithium cobaltate, which has high charging/discharging capacity per unit weight at high electric potential, and, as a negative electrode material, a carbon material, such as graphite, which exhibits high charging/discharging capacity per unit weight at a low electric potential nearly equal to that of Li. However, a gravimetric charging/discharging capacity of such an electrode material employed in the battery is consumed nearly equal to the theoretical value, and thus the gravimetric energy density of the battery is approaching its limit. Therefore, many attempts have been made to develop new positive electrode materials of high capacity, such as an iron olivine compound and a metal sulfide, and new negative electrode materials, such as a composite material formed of a carbon material, and tin oxide, silicon oxide, a Li alloy, or lithium nitride.
A secondary battery employed in a small-sized portable apparatus is required to have smaller size; i.e., not only high gravimetric energy density but also high volumetric energy density. Therefore, attempts have been made to increase the amount of an electrode material charged into a battery container by increasing the density of the electrode material, to thereby enhance the volumetric energy density of the resultant electrode and battery.
Graphite, which is most widely employed as a negative electrode material, has a true density of about 2.2 g/cm3, but currently available electrodes incorporating graphite have a density of about 1.5 g/cm3. When the density of the electrode employing graphite is increased to 1.7 g/cm3 or higher, the volumetric energy density of the resultant battery can be enhanced. Therefore, attempts have been made to increase the density of the electrode employing graphite. Meanwhile, lithium cobaltate, which is widely employed as a positive electrode material, has a true density of about 5.1 g/cm3, but currently available electrodes incorporating lithium cobaltate have a density of less than 3.3 g/cm3. Therefore, attempts have been made to increase the density of the electrode employing lithium cobaltate to 3.5 g/cm3 or higher.
However, as the density of an electrode is increased, the amount of pores contained in the electrode is reduced, leading to problems including shortage of the amount of an electrolytic solution which is present in the pores and plays an important role for electrode reaction and decrease in the permeation rate of the electrolytic solution throughout the electrode. As the amount of the electrolytic solution in the electrode is reduced, the rate of electrode reaction decreases, leading to problems such as lowering of energy density and high-speed charging/discharging performance, which further causes a problem that the cycling characteristics of the battery is lowered. Meanwhile, as permeability of the electrolytic solution is impaired, the time required for the production of a battery is lengthened, leading to an increase in production cost. Such problems become more pronounced in a case of a battery such as a lithium polymer battery, which employs a polymer compound as a part of the components or as whole component in the electrolytic solution.